1. Field of the Invention
The present invention relates to a mechanical date display mechanism, and to a timepiece, such as a wristwatch or a small clock, having the date display mechanism.
2. Description of the Prior Art:
A mechanical date display mechanism that has been widely adopted for a wristwatch or a small clock is one which displays a date in a date display window by rotating a date display plate, in which numerals of 0 to 31 have been printed in an outer circumference part of a disk having the same size as a dial, by a predetermined angle once in one day. For this reason, since a numeral displaying the date becomes smaller than a numeral displaying a time, there is a problem that a date display is difficult to see. Whereupon, a date display mechanism in which the date display is made large has been developed, and has been adopted in a wristwatch available in the market.
In Japanese Patent No. 3322678 Gazette, there is disclosed a date display mechanism possessing a date display window in a 12 o'clock position. This date display mechanism is constituted by a unit's place wheel which is a disc-like gear wheel in whose outer circumference part there have been printed unit's place numerals, i.e., numerals of 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, of date with an equal spacing, and which has been disposed such that the unit's place numeral of date appears in the date display window disposed in the 12 o'clock position, a ten's place wheel which is a disc-like gear wheel in whose outer circumference part there have been printed ten's place numerals, i.e., numerals of 0, 1, 2, 3, of date with an equal spacing, and which has been adapted such that the ten's place numeral appears in the date display window and has been disposed while being spaced such that its teeth don't mesh with the unit's place wheel, and a program wheel which is a doughnut-like gear wheel possessing 30 internal teeth for driving the unit's place wheel and 4 internal teeth for driving the ten's place wheel, and which is rotated by a date indicator driving wheel.
From 1st day to 31st day, the program wheel rotation-drives the unit's place wheel, and the unit's place wheel is unlocked by the 30 internal teeth for driving the unit's place wheel by for one tooth in one day. At the same time, the program wheel rotation-drives the ten's place wheel, and the ten's place wheel is unlocked by the 3 internal teeth for driving the ten's place wheel from the 1st to the 3rd by for one tooth in 10 days. Accordingly, the dates from 1st day to 31st day are displayed in date display window.
When it shifts from 31st day to 1st day, the unit's place wheel retains its settled position when it has shifted from 30th day to 31st day. This is achieved by not providing the internal tooth, of the program wheel, for driving the unit's place wheel between 1st day and 2nd day, but by making it into a skip interval. That is, even if the date indicator driving wheel unlocks the program wheel for one tooth, the unit's place wheel is retained in its settled position of 31st day. Accordingly, numeral of the unit's place appearing in the date display window is “1” intact. On the other hand, when it shifts from 31st day to 1st day, the ten's place wheel is unlocked for one tooth by the 4th internal tooth for driving the ten's place wheel. Accordingly, numeral of the ten's place appearing in the date display window is altered from “3” to “0”. In this manner, if it shifts from 31st day to 1st day, date appearing in the date display window is altered from “31” to “01”.
In short, the above-mentioned conventional date display mechanism is a date display mechanism constituted by the unit's place wheel formed monolithically with a unit's place display plate on which there have been printed numerals of the unit's place of date, the ten's place wheel formed monolithically with a ten's place display plate on which there have been printed numerals of the ten's place of date, and the program wheel which rotation-drives the unit's place wheel and the ten's place wheel in compliance with a program, characterized in that the program wheel is one which is unlocked every day by a predetermined angle by the date indicator driving wheel and thus rotated by one revolution in 31 days, and programmed such that the unit's place wheel is unlocked every day for one tooth from 1st day to 31st day and thus rotated by one revolution in 10 days, the ten's place wheel is unlocked by for one tooth respectively when it shifts from 9th day to 10th day, from 19th day to 20th day and from 29th day to 30th day, and additionally the unit's place wheel is retained to the position of 31st day and the ten's place wheel is unlocked by for one tooth when it shifts from 31st day to 1st day.
The above program, i.e., the program adapted such that the unit's place wheel is unlocked every day for one tooth from 1st day to 31st day and thus rotated by one revolution in 10 days, the ten's place wheel is unlocked by for one tooth when it shifts from 9th day to 10th day, from 19th day to 20th day and from 29th day to 30th day, and additionally the unit's place wheel is retained to the position of 31st day and the ten's place wheel is unlocked by for one tooth when it shifts from 31st day to 1st day, is implemented by a peculiar constitution of the internal teeth of the program wheel. The peculiar constitution of the internal teeth of the program wheel is realized by forming, in the program wheel, the 30 internal teeth for unlocking the unit's place wheel and one skip interval with an equal spacing, and the 4 internal teeth for driving the ten's place wheel. More concretely, the 30 internal teeth for unlocking the unit's place wheel and one skip interval are formed in a lower stage of the program wheel, and the 4 internal teeth for driving the ten's place wheel are formed in an upper stage of the program wheel.
Such a conventional date display mechanism as mentioned above is one which rotation-drives the unit's place wheel and the ten's place wheel by the program wheel having the 4 internal teeth for driving the ten's place wheel. For this reason, since it is necessary to prescribe 2 relative positions between the program wheel and the unit's place wheel and between the program wheel and the ten's place wheel, the above-mentioned conventional date display mechanism has been bad in its assembling property and has brought about an increase in assembling cost. Further, since it is necessary that the unit's place display plate is disposed coaxially with the unit's place wheel and the ten's place display plate with the ten's place wheel, respectively, there is also a problem that a maximum size of numeral appearing in the date display window is limited by a size of each of the unit's place wheel and the ten's place wheel.
A problem that the present invention is to solve is to provide a date display mechanism whose assembling property is high, and which has been constituted by a unit's place wheel which rotation-drives a unit's place display plate on which numerals of a unit's place of date have been printed, a ten's place wheel which rotation-drives a ten's place display plate on which numerals of a ten's place of date have been printed, and a program wheel which controls rotations of the unit's place wheel and the ten's place wheel.